The present invention relates to a method for laminating layers consisting at least in part of thermoplastic films.
In the case of known methods for laminating layers, and a corresponding double-plate pressing device for carrying out the method (European Patent Application No. 0013557), the layers are compressed to a composite material under the effect of heat and pressure. This is achieved by initially placing the blank, which is composed of the different layers, in the pressing device and applying two heatable plates on the two sides of the blank so as to exert heat and pressure upon both sides of the blank, and switching off the heating thereafter so that after cooling down the identification cards so produced can be removed from the pressing device.
In addition to disclosing the arrangement of resistance wires in the oversized pressing plates arranged on both sides, which interact directly with the layers to be laminated and which are screened from the supporting plates of the pressing device on which they are mounted by interposed insulating layers, this publication also discloses an arrangement where the pressing plates are configured as cooled supporting plates and are preceded by a flat heating resistance layer serving as a heating element, the heating resistance layer being arranged so as to directly face the layers to be laminated and being thermally screened against the cooled supporting plates by an insulating layer. The heating resistance layer is supplied with pulse-shaped voltages from a heater voltage source, it being an additional particularity of this known laminating device that certain, especially heat-sensitive security features, such as guilloche prints or watermarks, microfilm formers, holograms, or the like, can be provided in the check, identity or passport cards by means of dies arranged exclusively in the area of these additional pressure and temperature sensitive security features, which dies are set off to the rear and have a temperature lower than the temperature of the plates producing the heating effect. Consequently, the pressure in the area of the die openings is lower than the lamination pressure, and the temperature is also lower in these areas than the temperature of the plate.
The problem with all the laminating devices of the prior art lies in the fact that the production cycle consists of two clearly distinguishable subcycles, namely the initial application of heat, which should be as abrupt as possible to attain the laminating temperature of the layers of the identification card--only this term will be used hereafter to describe the object to be produced--that are to be fused, and a subsequent cooling cycle which is initiated by switching off the heating elements and during which the pressing device may not be opened to remove the laminated card blank because the latter still lacks the necessary mechanical stability.
It is, therefore, desirable to subject the blank, or initially the composite layers, in the pressing device at first to a heating step and then to a cooling phase, which means that the insulating layer arranged between the flat film heating layer and the pressing plate, which latter is actively cooled in most of the cases, has to fulfill two opposing functions both of which are performed by it unsatisfactorily: During the heating phase, it is to protect the adjacent cooling plates as perfectly as possible from the heat emitted or applied by the heating element, and to direct such heat to the layers to be laminated, while during the cooling phase, after the heating has been switched off, the heat flow, which previously was to be suppressed, is now to be enabled perfectly and comprehensively, and exactly in the direction toward the cooled supporting plates, i.e. that direction in which no heat flow was allowed during the preceding heating phase.
If, therefore, the "insulating layer" is designed to provide a highly heat-insulating effect, a good heating effect but unsatisfactory recooling is obtained, whereas the selection of a highly heat-conducting material for the "insulating layer" leads to satisfactory recooling, but impairs the initially required heat insulation.
The laminating device for producing identification cards known from European Patent Specification No. 0 154 970 also recognizes this problem mentioning that theretofore such laminating devices consisted of a heating element formed by glass fiber mats, arranged directly adjacent a metal block comprising heating fins, with resistance ladder paths arranged between the glass fiber mats. Thus, a considerable share of the heat initially produced flows into the metal block, thereby heating the latter, and cooling can be effected only after fusing of the card blanks, as otherwise the necessary fusing temperature will not be reached during the fusing process. Recooling of the heated mass of the metal block must take place only after completion of the fusing process. All this shows that with the cooling devices previously known the different phases of the before-mentioned cycles do not function properly, and that one therefore tries to make the disadvantages, which are always encountered, controllable by the use of a specifically designed time control. Yet, it cannot be avoided that a comparatively long total cycle (heating up and subsequent recooling) has to be accepted. In practice, such a cycle always takes between half of a minute and several minutes, and the heat losses are still very considerable, although the cited Patent Specification 0154 970 speaks, however, of a few seconds only.
Another publication describing a further known device (U.S. Pat. No. 4,108,713), which is also suited for laminating identification cards, provides a more detailed description of the flat heating element normally used for the lamination process. The heating element consists in this case of a carrier plate of small mass and correspondingly low thermal capacity so that both the heating process and the subsequent recooling process can be effected very quickly; the metallic carrier plate has a thickness of less than 0.25 mm, and an electric insulating layer consisting of silicone rubber is arranged between the flat heating element proper and the carrier plate. The flat heating element comprises flat resistance wires arranged in a meandering pattern, and a heat sensor is provided adjacent the resistance wires for heat control purposes.
In a different context, i.e. in connection with the production of thin thermoplastic synthetic resin films, a process is known (U.S. Pat. No. 4,752,204) where a resin block blank consisting of the respective synthetic resin is heated between a pair of heated pressing plates, while its dimensions are considerably changed under the effect of pressure in order to deform it into a film.
For this purpose, a suitable pressing tool consists of two oppositely arranged pressing jaws with pressing plates provided therein on both sides which pressing plates are held, under the effect of springs, at a certain distance from adjoining cooling blocks and are intended for receiving the synthetic resin block between them.
As one realized that it is not possible to cool and heat simultaneously, one decided in the case the pressing surfaces, which are kept at a distance from the cooling blocks, to first introduce between the surfaces a heating element, from a different location, for heating up the pressing surfaces on both sides. As soon as the surfaces are heated up sufficiently--the heat being initially prevented from dissipating through the existing spacing to the adjoining cooling blocks--the heating element is withdrawn, and the synthetic resin block to be deformed is inserted. Thereafter, the pressing dies are moved toward each other, and the heated pressing surfaces are pressed simultaneously against the cooling plates and the inserted synthetic resin block, while the previously active biasing springs give way, so that during the deforming process--and this is the only decisive aspect--the heat introduced is simultaneously transmitted to the blank and dissipated via the cooling blocks. Such a process may be suited for the production of films from thermoplastic synthetic resins, but is of no use for the production of a laminated composite material for identification cards, or the like, because it does not in any way remedy the real problem, namely that the heat is simultaneously introduced and dissipated during the processing step, i.e. when heat and pressure are exerted on the blank.
Another known method, which similarly belongs to a different class and which is intended for producing sandwich panels, makes use of intermediate products consisting of high-performance composite materials with polymeric matrices (German Offenlegungsschrift 39 10 021). Such sandwich elements comprise an inner honeycomb material, consisting for example of aluminium, a plastic material or impregnated paper, with polymers applied thereon as top laminates. The precut intermediate product, consisting of polymeric top layers and the core material, is initially heated up between two plates by resistance heating in pressureless condition, the pressure initially applied upon the two heating plates on both sides, via pressure springs and insulators, being just sufficient to ensure satisfactory heat transfer from the heating plates to the intermediate product.
Only when the processing temperature has been reached will the outer cooled pressing plates move toward the heating plates on both sides, and the heated intermediate product between them, so that during this phase the core material is compressed and united with the heated-up top layers, while heat is simultaneously dissipated from the heating plates toward the cooled die plates. It has to be assumed in this case that in practice the cooling action necessarily must be initiated before applying the pressing force required for the deformation process, as the pressing force can be applied only after the cooled pressing plates are in full contact with the extremely thin heating plates. This means that in the case of this known method, too, at least cooling and deformation take place simultaneously.
Now, it is the object of the present invention to remedy this situation and to provide a method for producing identification cards, credit cards, and the like, which reconciles and complies with the fundamentally opposing requirements of ensuring at the same time efficient thermal insulation (during the heating process), followed by rapid and perfect, unobstructed cooling of the laminated product.